1. Field of the Invention
The present invention relates to a piezoelectric vibrating reed, and a piezoelectric vibrator, an oscillator, an electronic device, and a radio-controlled timepiece each using the piezoelectric vibrating reed.
2. Description of the Related Art
Piezoelectric vibrators using crystal or the like are used in mobile phones or mobile information terminals as a time source, a control signal timing source, a reference signal source, and the like. Various piezoelectric vibrators are available as such kinds of piezoelectric vibrators, and a piezoelectric vibrator having a tuning-fork type piezoelectric vibrating reed is also known as one of the piezoelectric vibrators.
However, in recent years, with miniaturization of devices on which the piezoelectric vibrating reed is mounted, a further reduction in the size of the piezoelectric vibrating reed is desired. As a method of realizing size reduction of the piezoelectric vibrating reed while suppressing the CI value (Crystal Impedance) of a piezoelectric vibrator to a low value, a method of forming groove portions on both principal surfaces of a vibrating arm portion of the piezoelectric vibrating reed is known (for example, see JP-A-2002-76827).
FIG. 14 is a view illustrating the related art and is also a cross-sectional view along the surface vertical to the longitudinal direction of a vibrating arm portion 203 disclosed in JP-A-2002-76827. As shown in FIG. 14, a first excitation electrode 210 is mainly formed inside a groove portion 205, a second excitation electrode 211 is formed on the side surfaces of the vibrating arm portion 203. The vibrating arm portion 203 has an approximately H-shaped cross-section.
In general, the CI value of a piezoelectric vibrating reed in which the groove portion 205 is formed is known to depend on the width D between the side surface 203a of the vibrating arm portion 203 and the side surface 205a of the groove portion. Specifically, the CI value can be suppressed to a low value by narrowing the width D.
Moreover, in order to realize high performance of the piezoelectric vibrating reed, it is important to suppress vibration leakage and secure favorable driving level characteristics.
Here, vibration leakage means leakage of vibration energy in which vibration of a piezoelectric vibrating reed leaks to the outside when the piezoelectric vibrating reed is operated. Since the vibration leakage leads to degradation of efficiency of the piezoelectric vibrating reed, it is necessary to suppress the vibration leakage as much as possible.
Moreover, the driving level characteristics means the variation characteristics of the vibration frequency of the piezoelectric vibrating reed with respect to variation of a driving voltage. The smaller the variation Δf of the vibration frequency, the more favorable the driving level characteristics and the higher the performance of the piezoelectric vibrating reed.
In general, the vibration leakage and driving level characteristics of the piezoelectric vibrating reed are known to depend on the rigidity of the vibrating arm portion 203. Moreover, the rigidity of the vibrating arm portion 203 depends on the area S (hereinafter referred to as a “cross-sectional area S of a vibrating arm portion”) of a cross-section vertical to the longitudinal direction of the vibrating arm portion 203. Thus, the vibration leakage and the driving level characteristics depend on the cross-sectional area S of the vibrating arm portion 203. Specifically, by increasing the cross-sectional area S of the vibrating arm portion 203 to increase the rigidity of the vibrating arm portion 203, it is possible to suppress the vibration leakage and to secure favorable driving level characteristics.
However, a piezoelectric vibrating reed disclosed in JP-A-2002-76827 includes a base portion in which a base electrode portion (corresponding to the “mount electrode” of the present disclosure), a vibrating narrow rod (corresponding to the “vibrating arm portion” of the present disclosure) formed so as to protrude from the base portion, a groove portion having a groove electrode portion (corresponding to an “excitation electrode” of the present disclosure) formed on the top surface and/or the bottom surface of the vibrating narrow rod, and a connection electrode portion (corresponding to an “extraction electrode” of the present disclosure) connecting the base electrode portion and the groove electrode portion to each other. The width on the base end side of the groove portion is narrower than the other portions, and a large region for forming the connection electrode portion is secured. In the JP-A-2002-76827, since the cross-sectional area S on the base end side of the vibrating arm portion 203 is increased, as a result, the rigidity on the base end side of the vibrating arm portion is increased.
However, in the JP-A-2002-76827, since the rigidity of only the base end side of the vibrating arm portion is increased, it is difficult to secure sufficient rigidity of the entire vibrating arm portion 203. Thus, it may be difficult to suppress the vibration leakage and to secure favorable driving level characteristics. Moreover, on the base end side, since the width D (see FIG. 14) between the side surface of the vibrating arm portion and the side surface of the groove portion is increased, the CI value may increase.